Loader README update, resolves #570

The new description was taken from here: http://yuilibrary.com/yui/docs/yui/loader.html

The `Development` and `Testing` section were deleted, because learning how to build and test a module should best be left in some other document, such as `Developer Workflow`, located [here] (https://github.com/yui/yui3/wiki/Developer-Workflow). And not to mention the information was extremely outdated.

Also, every other module's `README` in YUI never goes into detail as to how to build or test it. Their `README`s always provide a basic description of what the module does, and nothing more.

Examples:
 * [Anim](https://github.com/yui/yui3/tree/master/src/anim)
 * [DOM](https://github.com/yui/yui3/tree/master/src/dom)
 * [Widget](https://github.com/yui/yui3/tree/master/src/widget)

This resolves #570.